Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 41805)
+++ channels/chan_sip.c	(working copy)
@@ -9745,10 +9745,31 @@
 {
 	int expires, expires_ms;
 	struct sip_registry *r;
+	char via[1024] = "";
 	r=p->registry;
 
 	switch (resp) {
 	case 401:	/* Unauthorized */
+		/* automatically set IP address if behind NAT router */
+				
+
+				memset(via, 0, sizeof(via));
+				strncpy(via, get_header(req, "Via"), sizeof(via) - 1);
+				char *c = strstr(via, "received=");
+				if (c) {
+					char *received = c + 9;
+					struct hostent *hp;
+					struct ast_hostent ahp;
+					c = strchr(received, ';');
+					if (c) 
+						*c = '\0';
+					ast_log(LOG_NOTICE, "REGISTER indicated my host name is %s\n", received);
+					if (!(hp = ast_gethostbyname(received, &ahp))) 
+						ast_log(LOG_WARNING, "Invalid address for in received= in Via: %s\n",received);
+					else
+						memcpy(&externip.sin_addr, hp->h_addr, sizeof(externip.sin_addr));
+				}
+				
 		if ((p->authtries == MAX_AUTHTRIES) || do_register_auth(p, req, "WWW-Authenticate", "Authorization")) {
 			ast_log(LOG_NOTICE, "Failed to authenticate on REGISTER to '%s@%s' (Tries %d)\n", p->registry->username, p->registry->hostname, p->authtries);
 			ast_set_flag(p, SIP_NEEDDESTROY);